This invention relates to electrolytic cells suited for the electrolysis of aqueous solutions. More particularly, this invention relates to electrolytic cells suited for the electrolysis of aqueous alkali metal chloride solutions.
Electrolytic cells have been used extensively for many years for the production of chlorine, chlorates, chlorites, caustic, hydrogen and other related chemicals. Over the years, such cells have been developed to a degree whereby high operating efficiencies have been obtained, based on the electricity expended. Operating efficiencies include current, voltage and power. The most recent developments in electrolytic cells have been in making improvements for increasing the production capacities of the individual cells while maintaining high operating efficiencies. This has been done to a large extent by modifying or redesigning the individual cells and increasing the current capacities at which the individual cells operate. The increased production capacities of the individual cells operating at higher current capacities provide higher production rates for given cell room floor areas and reduce capital investment and operating costs. In general, the most recent developments in electrolytic cells have been towards larger cells which have high production capacities and which are designed to operate at high current capacities while maintaining high operating efficiencies. Within certain operating parameters, the higher the current capacity at which a cell is designed to operate, the higher is the production capacity of the cell. As the designed current capacity of a cell is increased, however, it is important that high operating efficiencies be maintained. Mere enlargement of the component parts of a cell designed to operate at low current capacity will not provide a cell which can be operated at high current capacity and still maintain high operating efficiencies. Numerous design improvements must be incorporated into a high current capacity cell so that high operating efficiencies can be maintained and high production capacity can be provided.
The development in electrolytic cells is demonstrated by Table 1:
______________________________________ Amperage kA 80 150 200 Number of Anodes per cell 42 75 100 Number of rows per cell 2 3 4 Anodes per row 21 25 25 Approx. cell width (m) 1.6 2.3 3.0 Approx. cell length (m) 1.9 2.2 2.2 Aspect ratio 1.2 1.0 0.7 Amperage kA/m 42 68 91 per m cell length chlorine production (tons/day) 2.4 4.5 6.0 ______________________________________
It is known to perform the electrolysis of aqueous solutions on an industrial scale in cells equipped with either horizontal electrodes sloping towards the horizontal plane of the floor, or with vertical electrodes.
This invention describes a novel cell with vertical electrodes. Cells with vertical electrodes are composed of at least one anode and one cathode, preferably, however, of a plurality of anodes and cathodes, the active anode and cathode surfaces being substantially arranged vertically and in parallel to each other. The gap between each anode and cathode surface is filled with the electrolyte.
An important field of application of cells with vertical electrodes is, for example, the electrolytic production of chlorine, caustic soda and hydrogen from alkali metal chlorides. For this field of application, a separator must be provided in the electrolysis space between anode and cathode surfaces. This separator is required to provide little obstruction to the ion transport necessary for the electrolysis while substantially avoiding any mixing of the products formed on the electrode surfaces. Various materials are known to possess the properties required to provide the proposed purpose of the separator for the alkali metal chloride electrolysis process. Use is made, for example, of asbestos as well as of different microporous plastics materials or nonporous ion exchange materials.
A basic requirement for any electrolysis cell is to maintain at a minimum the electrolysis gap, i.e., the space between anode and cathode surface, because energy losses will rise significantly with increased electrode spacing, because of the high electrical resistance of the electrolyte.
In the early prior art, chlor-alkali diaphragm cells were designed to operate at the above mentioned current capacities having the shown production capacities. Inasmuch as the production rate of electrolysis cells is limited, industrial plants comprise a plurality of cells connected in series electrically. Bus-bars made of a material of good electrical conductivity, for example, copper or aluminum, are used for the electrical connection of the cells. The specific load, i.e. current density per unit of cross-sectional area, of these bus-bars is subject to limitation, because physical laws teach that the temperature of an electric conductor is bound to rise as the specific load increases, and also the energy loss through the conductor resistance will increase. As electrolysis cells are operated at high current capacities, the cross-sectional areas of the bus-bars must be sized accordingly. For as an instance at a load of 200 kA, the total cross-sectional area of the bus-bars of each cell connection would have to be about 1,000 sq. cm for copper bus-bars.
Within the electrolysis cell, the electrical connection from the bus-bars to the anode and cathode surfaces is made by an anode and cathode structure, that is also fabricated of materials of good electrical conductivity.
For the reason outlined above, the cross-sectional areas of the anode and cathode structures must also be adapted to the current load of the cell. As the total expense of conductive material results from the product of conductor cross-sectional area and conductor length, while the conductor cross-sectional area for a given cell load is fixed for said reasons, it is another basic requirement for electrolysis cells that the total conductor length of the cell plant be reduced as far as possible for limitation of conductor material expense.
In conventional plants, this is achieved by arranging the cells in a row and reducing the spacing of the cells within a row. Basically, this principle of the shortest current path is characterized by the fact that the reduction of conductor material expense and electrical energy losses requires the reduction of the spacing between centerlines of adjacent electrolysis cells arranged in one row.
One way to reduce the spacing of centerlines of adjacent cells is to hold the free space between adjacent cells at a minimum. This method is common practice in conventional electrolysis plants. The spacing between centerlines of electrolysis cells can also be limited by reducing the cell width, i.e., the extension of the cell in the direction of the cell row as shown in FIG. 1,2,& 3. As a certain definite number of electrode elements must be installed for maintaining the conventional production rate of a cell, while the space occupied by these elements corresponds to the product of cell width and cell length, (cell lengths shall be construed to mean the extension of the cell perpendicular to the direction of the cell row as shown in FIG. 1,2& 3) the cell length must be extended inversely proportional to any reduction of cell width.
The principle of the shortest current path thus leads to the demand to design the electrolysis cells in such a way that the aspect ratio of cell length/cell width be as large as possible.
Cells with horizontal or sloping electrodes do not present any major difficulties to be designed for a large aspect ratio.
Many types of the known mercury cells used for the production of chlorine and NaOH have been designed with an aspect ratio of 8 through 10 or even more.
Referring to the known types of cells with vertical electrodes, however, and particularly referring to the known diaphragm cells for the production of chlorine and NaOH, the cell design is either a square or a relatively wide rectangle with an aspect ratio of approx. 1 to 2.
For cells with vertical electrodes, increasing this aspect ratio to any considerable extent would present basic difficulties.
More anode and cathode elements must be installed alternately in series in a longitudinal direction of the cell as the cell length is increased.
As the same time, the spacing between adjacent anode and cathode elements must be held at a minimum as outlined above.
Because the anode part and the cathode part of a cell are fabricated in separate production process, mostly even in different works, and because each fabrication process is bound to require non-avoidable dimensional tolerances, full dimensional conformity between anode and cathode parts cannot be achieved.
As each individual element of anode and cathode parts is already subject to dimensional tolerances, the total deviation of anode and cathode parts from the theoretical dimension will necessarily increase with the number of electrode elements arranged in series. The increasing deviation from the theoretical dimension of anode and cathode parts at increasing cell length might lead to a considerable difference of the distance between an anode part and an adjacent cathode part during the assembly of both parts. This will in any case adversely affect the electrolysis process; further the spacing might become so small that there is no space left for the separator or that anode and cathode parts will come in contact during assembly.
A further limitation regarding current load and production rate of conventional cells with vertical anodes is caused by the strong magnetic fields in the cell area, which exert considerable forces upon all cell parts made of magnetic material, such as iron, steel, stainless steel, etc. These magnetic forces might seriously disturb the operation of an electrolysis plant. At the time of replacing a cell, for example, the crane is not only loaded with the cell itself, but has also to over come considerable magnetic forces developed from the adjacent cells. Further the cell suspended on the crane would tend to orientation with the gradient of the magnetic field, which would lead to unforeseeable and dangerous movements of the cell. In addition, any parts made of magnetic material, such as screws, bolts, clamps, piping joints, etc., can only be mounted and dismantled on cells subjected to strong magnetic forces after taking adequate safety precautions.